Major Histocompatibility Complex (MHC) class I glycoproteins, the products of HLA-A, B and C genes in humans, are critical for T cell mediated immune responses to viruses and intracellular pathogens. They are glycoproteins that form a heterodimer with a small molecule, ?2-microglobulin (?2m), and associate in the endoplasmic reticulum (ER) of a cell with peptides derived from cellular proteins. These include, when the cell is infected, pathogen-encoded proteins. Surface complexes of MHC class I molecules with pathogen-derived peptides are recognized by CD8-positive T cells that can kill the infected cell. The goal of this proposal is to understand the detailed molecular processes that result in the formation and cell surface expression of MHC class I complexes that contain peptides of extraordinarily high affinity, which is essential for CD8-T cell responses that can rid an infected individual of the pathogen. Binding of peptides to MHC class I molecules occurs within a multi-protein assembly called the Peptide Loading Complex, or PLC. The PLC consists of MHC class I molecules themselves, a heterodimeric transporter called the Transporter associated with Antigen Processing (TAP) that delivers the peptides into the ER, tapasin, a membrane glycoprotein that, with a soluble molecule, ERp57, forms a disulfide-linked heterodimer that can mediate peptide exchange by the class I molecules to ultimately generate high affinity complexes, and a second soluble protein called calreticulin. The interactions responsible for the stability of the PLC involve specific associations of TAP and tapasin within the membrane, and between the luminal domains of tapasin and the MHC class I molecule. There are also interactions between ERp57 and calreticulin and a glycan-dependent interaction of the MHC class I glycoprotein with calreticulin that are shared by other folding glycoproteins in the ER. Th glycan structure is characteristically dynamically maintained by the action of two opposing enzymes, one, glucosidase II, that removes a terminal glucose residue, and a second, UDP-glucose glycoprotein transferase 1 (UGT1) that replaces the glucose when the glycoprotein bearing the glycan is improperly folded. This proposal seeks to determine the roles of these various interactions and enzymatic mechanisms in mediating the assembly of MHC class I molecules with high affinity peptides in the ER.